U.S. patent number 5,206,925 [Application Number 07/692,336] was granted by the patent office on 1993-04-27 for rare earth element-doped optical waveguide and process for producing the same.
This patent grant is currently assigned to Hitachi Cable Limited, Nippon Telegraph and Telephone Corporation. Invention is credited to Katsuyuki Imoto, Toshikazu Kamoshida, Seiichi Kashimura, Fujio Kikuchi, Yasuo Kimura, Masataka Nakazawa.
United States Patent |
5,206,925 |
Nakazawa , et al. |
April 27, 1993 |
Rare earth element-doped optical waveguide and process for
producing the same
Abstract
A core waveguide having a substantially rectangular cross
section, with the width thereof greater than the thickness thereof,
is provided in a cladding formed on a substrate, and a rare earth
element-doped layer is provided in the core waveguide along the
waveguiding direction of the waveguide. With the width of the core
waveguide set greater than the thickness of the core waveguide,
good optical confinement in the width direction of the waveguide is
obtained, which enables light to be absorbed by the rare earth
element-doped layer efficiently and concentratedly. It is thereby
possible to achieve a marked improvement in the excitation
efficiency of excitation power. Thus, an enhanced excitation
efficiency is achieved with less addition of a rare earth element,
and a high-gain optical amplification waveguide free of
concentration extinction is provided.
Inventors: |
Nakazawa; Masataka (Mito,
JP), Kimura; Yasuo (Mito, JP), Imoto;
Katsuyuki (Sayama, JP), Kashimura; Seiichi
(Hitachi, JP), Kamoshida; Toshikazu (Kuji,
JP), Kikuchi; Fujio (Katsuta, JP) |
Assignee: |
Hitachi Cable Limited (Tokyo,
JP)
Nippon Telegraph and Telephone Corporation (Tokyo,
JP)
|
Family
ID: |
15955142 |
Appl.
No.: |
07/692,336 |
Filed: |
April 26, 1991 |
Foreign Application Priority Data
|
|
|
|
|
Jun 29, 1990 [JP] |
|
|
2-173156 |
|
Current U.S.
Class: |
385/142; 216/24;
385/130; 385/131; 385/132; 385/14 |
Current CPC
Class: |
C03C
13/045 (20130101); H01S 3/063 (20130101) |
Current International
Class: |
C03C
13/00 (20060101); C03C 13/04 (20060101); H01S
3/063 (20060101); H01S 3/06 (20060101); G02B
006/10 (); H01L 021/70 () |
Field of
Search: |
;350/96.12,96.29,96.30,96.31,96.32,96.33,96.34
;385/123-132,144,142,144,14 ;437/51,15 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0281800 |
|
Sep 1988 |
|
EP |
|
0304709 |
|
Mar 1989 |
|
EP |
|
1443750 |
|
Jul 1976 |
|
GB |
|
2113006 |
|
Jul 1983 |
|
GB |
|
2180667 |
|
Apr 1987 |
|
GB |
|
2181861 |
|
Apr 1987 |
|
GB |
|
2181862 |
|
Apr 1987 |
|
GB |
|
2223351 |
|
Apr 1990 |
|
GB |
|
Primary Examiner: Healy; Brian
Attorney, Agent or Firm: Gossett; Dykema
Claims
We claim:
1. A rare earth element-doped optical waveguide comprising:
a substrate;
a cladding provided on the substrate;
a core waveguide provided in the cladding, the core waveguide being
substantially rectangular in cross section with the width thereof
greater than the thickness thereof; and
a rare earth element-doped layer provided in the core waveguide
along a waveguiding direction, whereby good optical confinement is
ensured in the width direction of the waveguide.
2. The optical waveguide of claim 1, wherein the rare earth
element-doped layer is provided at a substantially central portion
in the thickness direction of the core waveguide, and the rare
earth element-doped layer has a uniform impurity doping
concentration in the width direction thereof.
3. The optical waveguide of claim 1, wherein a plurality of the
rare earth element-doped layers are provided and the layers are
spaced from each other in the thickness direction of the core
waveguide.
4. The optical waveguide of claim 3, wherein at least one of the
rare earth element-doped layers has a thickness different from a
thickness of another rare earth element-doped layer.
5. The optical waveguide of claim 4, wherein the volume occupied by
the rare earth element-doped layers is not more than 30% of the
entire volume of the core waveguide.
6. The optical waveguide of claim 4, wherein the refractive index
of the rare earth element-doped layer is greater than the
refractive index of that region of the core waveguide which is not
doped with a rare earth element.
7. A rare earth element-doped optical waveguide comprising:
a substrate;
a cladding provided on the substrate; and
a core waveguide provided in the cladding, the core waveguide being
substantially rectangular in cross section, the core waveguide
including a first core waveguide doped with a rare earth element
and a second core waveguide not doped with a rare earth element,
the first core waveguide being substantially rectangular in cross
section, the second core waveguide covering the first core
waveguide.
8. The optical waveguide of claim 7, wherein the first core
waveguide is doped with the rare earth element in a layer pattern
along a waveguiding direction.
9. The optical waveguide of claim 8, wherein the refractive index
of the first core waveguide is equal to or greater than the
refractive index of the second core waveguide.
10. A method of producing a rare earth element-doped optical
waveguide comprising the steps of;
providing a first cladding layer on a substrate;
laminating on the first cladding layer alternately at least one
core layer not doped with a rare earth element and at least another
core layer doped with a rare earth element;
forming on the first cladding layer a core waveguide having a rare
earth element-doped layer and being substantially rectangular in
cross section; and
providing a second cladding layer so as to cover the entire surface
of the core waveguide.
11. The method of producing a rare earth element-doped optical
waveguide comprising the steps of:
providing a first core waveguide doped with a rare earth element
and substantially rectangular in cross section on a substrate
having a low refractive index;
providing a core layer not doped with a rare earth element over the
substrate so as to bury completely the first core waveguide
therein;
forming on the substrate a core waveguide including the first core
waveguide and a rare earth element-undoped second core waveguide
covering the first core waveguide; and
providing a cladding layer so as to cover the entire surface of the
core waveguide.
12. The method of claim 11, wherein a core layer formed of the same
material as the second core waveguide is provided on the substrate,
and the first core waveguide is provided on the thus formed core
layer.
Description
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to an optical waveguide for optical
amplification, particularly a rare earth element-doped optical
waveguide having a core waveguide doped with a rare earth element,
and to a process for producing the same.
2. Background Art
Optical fiber amplifiers and fiber lasers in which the core layer
of an optical fiber is doped with a rare earth element, such as Er
(for amplification at wavelengths around 1.55 .mu.m) and Nd (for
amplification at wavelengths around 1.3 .mu.m), are vigorously
studied at present for use thereof as optical amplifiers. The
optical fiber amplifiers and fiber lasers have the advantages that
(1) the core diameter thereof as small as 10 .mu.m ensures an
enhanced excitation power density, leading to a higher excitation
efficiency, (2) they permit a longer interaction length, (3) they
show a very low loss when a silica optical fiber is used therein,
and so on.
However, when the optical fiber amplifiers and fiber lasers are
mounted together with semiconductor lasers, photodetection devices,
optical modulation circuits, optical branching/coupling circuits,
optical switching circuits, optical wave mixing/separating circuits
or the like to construct a system, there arises the problem that,
because of the discrete component parts, it is difficult to obtain
a system with a smaller size and a lower loss. In addition, the
discrete component parts should be arranged with adjustments of the
respective optical axes of the component parts. The adjustments
require a very long time, leading to a higher cost, and bring about
reliability problems.
Recently, therefore, attention has come to be paid to rare earth
element-doped silica optical waveguides (the rare earth element
being Er or Nd in most cases) for their potential use as
future-type optical amplifiers, in view of the probability that the
doped silica optical waveguides can be made smaller and integrated,
as contrasted to the optical fiber type amplifiers.
There has been known a process for producing a silica optical
waveguide as shown in FIG. 12 (K. Imoto, et al., "Guided-wave
multi/demultiplexer with high stopband rejection", Applied Optics
Vol. 26, No. 19, October 1987, pp. 4214-4219). The process
comprises a series of the following steps (1) to (8):
(1) providing a core glass film 25 (SiO.sub.2 -TiO.sub.2 glass) on
a substrate 1 (SiO.sub.2 glass) [FIG. 12(a)], with the
refractive-index difference between the glass film 25 and the
substrate 1 being about 0.25% and with the thickness T of the glass
film 25 being about 8 .mu.m;
(2) heat-treating the thus obtained assembly at a high temperature
of about 1200.degree. C. to make the film 25 denser [FIG.
12(b)];
(3) providing a WSi.sub.x film 26 which, to be used for etching the
core glass film 25, is about 1 .mu.m thick [FIG. 12(c)];
(4) applying a photoresist to the WSi.sub.x film 26 and patterning
the thus formed photoresist film 27 by photolithography [FIG.
12(d)];
(5) patterning the WSi.sub.x film 26 by dry etching, with the
patterned photoresist film 27 as a mask [FIG. 12(e)];
(6) patterning the core glass film 25 by dry etching, with the
patterned photoresist film 27 and the patterned WSi.sub.x film 26
as a mask [FIG. 12(f)];
(7) removing the photoresist film 27 and the WSi.sub.x film 26
[FIG. 12(g)]; and
(8) providing a clad layer 28 (SiO.sub.2 -P.sub.2 O.sub.5 -B.sub.2
O.sub.3 glass) over the substrate 1 so as to cover the patterned
core glass film 25, thereby producing a silica optical waveguide
having a substantially rectangular shaped core waveguide 3 in the
cladding.
It is difficult for an optical waveguide with a planar structure to
be formed into an elongate shape, as in the case of an optical
fiber. To obtaining a better excitation efficiency, therefore, an
increased amount of a rare earth element should be added to the
core of the optical waveguide. It has been found, however, that
doping with a large amount of a rare earth element causes
concentration extinction, thereby making it impossible to obtain
the desired lasing or amplifying function.
The above-mentioned conventional process for producing a silica
optical waveguide can be used, with no special problems, where the
core glass film 25 is not doped with a rare earth element such as
Er and Nd. It has been found, however, that where the core glass
film 25 is doped with a rare earth element the conventional process
results in roughening of side surfaces of the core waveguide in the
step of patterning the core glass film 25 by dry etching, shown in
FIG. 12(f). The roughened side surfaces of the core waveguide cause
scattering of the propagating light, leading to an energy loss and
a lowered optical amplification efficiency. FIG. 13(a) shows an SEM
photograph of a rare earth element-doped core waveguide formed by
the conventional process, and FIG. 13(b) shows an SEM photograph of
a core waveguide not doped with a rare earth element. The
photographs show how the side surfaces of the core waveguide doped
with a rare earth element is roughened, and also show the
deposition of a product, which is considered to be a compound of
the rare earth element, on the surfaces of the waveguide.
This is because the rare earth element added to the core glass film
25 is left unetched after the step of patterning the core glass
film 25 by dry etching. For instance, when an Er-doped core glass
film is dry etched by use of CHF.sub.3 as a reactive gas, Si and Ti
are converted into reaction products of high vapor pressures
through the reactions:
and are thereby etched away. On the other hand, Er is converted
into a reaction product of a low vapor pressure through the
reaction process:
and the reaction product ErF.sub.3 is left unetched. The symbol "*"
is used here to indicate that the same discussion applies to the
cases where a chlorine-containing etching gas other than CHF.sub.3
is used.
SUMMARY OF THE INVENTION
It is accordingly an object of this invention to provide a rare
earth element-doped optical waveguide capable of displaying an
optical amplification function with high efficiency and a process
for producing the same.
According to a first aspect of this invention, there is provided a
rare earth element-doped optical waveguide in which a core
waveguide substantially rectangular in cross section with the width
thereof greater than the thickness thereof is provided in a
cladding formed on a substrate, and a rare earth element-doped
layer is provided in the core waveguide along a waveguiding
direction. With the width of the core waveguide set greater than
the thickness of the core waveguide, good optical confinement in
the width direction of the waveguide is ensured, which enables the
light to be absorbed by the rare earth element-doped layer
efficiently and concentratedly. Consequently, it is possible to
achieve a marked improvement in the excitation efficiency of
excitation power. It is thus possible to achieve a higher
excitation efficiency through doping with a smaller amount of a
rear earth element, and to realize a high-gain optical
amplification waveguide free of concentration extinction.
According to a second aspect of this invention, there is provided a
rare earth element-doped optical waveguide in which the
above-mentioned rare earth element-doped layer is formed at a
substantially central portion of the core waveguide, with respect
to the thickness direction of the core waveguide, and has a uniform
impurity doping concentration with respect to the width direction
thereof. With the rare earth element-doped layer thus provided at
the substantially central portion with respect to the thickness
direction of the core waveguide, at which portion the power
distribution of the excitation light reaches its maximum, it is
possible to effectively enhance further the excitation efficiency.
Thus, a higher excitation efficiency is achievable through doping
with a small amount of a rare earth element, and it is possible to
obtain an high-gain optical amplification waveguide free of
concentration extinction.
According to a third aspect of this invention, there is provided a
rare earth element-doped optical waveguide in which a plurality of
the rare earth element-doped layers are provided in the core
waveguide in the state of being spaced from each other along the
thickness direction of the core waveguide. The arrangement of the
plurality of rare earth element-doped layers in correspondence with
the power distribution of excitation light in the core waveguide
promises a further effective improvement of the excitation
efficiency. It is thus possible to enhance the excitation
efficiency with a small addition of a rare earth element, and to
realize a high-gain optical amplification waveguide which does not
cuase concentration extinction. Moreover, the arrangement of the
plurality of rare earth element-doped layers enables an optical
circuit with less dependency on polarization.
According to a fourth aspect of this invention, there is provided a
rare earth element-doped optical waveguide in which at least one of
the plurality of the rare earth element-doped layers provided
spaced apart from each other along the thickness direction of the
core waveguide has a thickness different from the thickness of each
of the other rare earth element-doped layers. In providing a
plurality of rare earth element-doped layers in accordance with the
power distribution of excitation light in the core waveguide, the
rare earth element-doped layer located at a portion where the power
is more concentrated than at other portions is thus set thicker
than the other doped layers, whereby a further effective
enhancement of the excitation efficiency is enabled. Thus, a higher
excitation efficiency is achieved through doping with a small
amount of a rare earth element, and it is possible to realize a
high-gain optical amplification waveguide free of concentration
extinction. In addition, the arrangement of the plurality of rare
earth element-doped layers offers an optical circuit which is less
dependent on polarization.
According to a fifth aspect of this invention, there is provided a
rare earth element-doped optical waveguide in which the volume
occupied by the region of the rare earth element-doped layer(s) is
not more than 30% based on the entire volume of the core waveguide.
Where a plurality of rare earth element-doped layers are provided,
concentration extinction will not occur if the volume occupied by
the rare earth element-doped layers is not more than 30% based on
the entire volume of the core waveguide. It is thus possible to
realize a high-gain optical amplification waveguide.
According to a sixth aspect of this invention, there is provided a
rare earth element-doped optical waveguide in which the refractive
index of the rare earth element-doped layer is greater than the
refractive index of the region of the core waveguide which is not
doped with a rare earth element. With the refractive index of the
rare earth element-doped layer set higher, light is confined in the
rare earth element-doped layer and is absorbed by the rare earth
element-doped layer efficiently and concentratedly. This enables a
marked increase in the excitation efficiency of excitation power.
Thus, a higher excitation efficiency is achieved through doping
with a small amount of a rare earth element, and it is possible to
realize a high-gain optical amplification waveguide free of
concentration extinction.
According to a seventh aspect of this invention, there is provided
a rare earth element-doped optical waveguide comprising a core
waveguide substantially rectangular in cross section provided in a
cladding formed on a substrate, in which the core waveguide
comprises a first core waveguide doped with a rare earth element
and substantially rectangular in cross section, and a second core
waveguide provided so as to cover the first core waveguide and not
doped with a rare earth element. The arrangement in which the first
core waveguide doped with a rare earth element is covered by the
second core waveguide not doped with a rare earth element enables
prevention of the scattering of the propagating (transmitted) light
at the surface of the first core waveguide. It is thus possible to
reduce the energy loss arising from scattering of the propagating
(transmitted) light, and to enhance optical amplification
efficiency.
According to an eighth aspect of this invention, there is provided
a rare earth element-doped optical waveguide in which the first
core waveguide is doped with the rare earth element in a layer form
along the waveguiding direction. The arrangement in which the first
core waveguide doped with the rare earth element in a layer form is
covered by the second core waveguide not doped with a rare earth
element ensures that the propagating (transmitted) light is
prevented from scattering at the surface of the rare earth
element-doped portion of the first core waveguide. It is thus
possible to reduce the energy loss arising from scattering of the
propagating (transmitted) light, and to enhance the excitation
efficiency through doping with a small amount of a rare earth
element. Consequently, a high-gain optical amplification waveguide
free of concentration extinction is achievable.
According to a ninth aspect of this invention, there is provided a
rare earth element-doped optical waveguide in which the refractive
index of the first core waveguide is equal to or greater than the
refractive index of the second core waveguide. With the refractive
index of the first core waveguide thus set equal to or greater than
the refractive index of the second core waveguide, better optical
confinement in the first core waveguide is ensured, and the light
is absorbed by the rare earth element-doped layer efficiently and
concentratedly. It is thus possible to enhance the transmission
efficiency of the core waveguide, and to enhance markedly the
excitation efficiency of excitation power.
According to a tenth aspect of this invention, there is provided a
process for producing a rare earth element-doped optical waveguide
comprising the steps of providing a first clad layer on a
substrate, laminating alternately at least one core layer not doped
with a rare earth element and at least one core layer doped with a
rare earth element, on the first clad layer, carrying out
photolithography, dry etching and the like to form a core waveguide
having a rare earth element-doped layer and a substantially
rectangular cross-sectional shape on the first clad layer, and
providing a second clad layer so as to cover the entire surface of
the core waveguide. Since the rare earth element-doped optical
waveguide is formed on the substrate by the planar technique, a
predetermined portion of the core waveguide constituting a
light-transmitting portion of the glass waveguide is capable of
being doped with the rare earth element concentratedly and
uniformly with respect to the width direction. It is thus possible
to enhance the excitation efficiency through "doping with a small
amount of a rare earth element, and to realize a high-gain optical
amplification waveguide which hardly exhibits any concentration
extinction. Further, this process enables the rare earth
element-doped optical waveguide to be formed together with other
optical devices on the substrate in a collective manner; therefore,
it is possible to produce a multifunctional system with high
quality.
According to an eleventh aspect of this invention, there is
provided a process for producing a rare earth element-doped optical
waveguide comprising the steps of providing a first core waveguide
doped with a rare earth element and substantially rectangular in
cross section on a substrate having a low refractive index,
providing a core layer not doped with a rare earth element over the
substrate so as to bury completely the first core waveguide,
carrying out photolithography, dry etching and the like to form on
the substrate a core waveguide comprising the first core waveguide
and a rare earth element-undoped second core waveguide covering the
first core waveguide, and providing a clad layer so as to cover the
entire surface of the core waveguide. Thus, in providing the core
waveguide, the first core waveguide doped with the rare earth
element is formed and then the rare earth element-undoped second
core waveguide covering the first core waveguide is formed, whereby
the surface of the core waveguide is formed to be smooth. This
enables prevention of the scattering of the propagating
(transmitted) light at the surface of the core waveguide, thereby
making it possible to reduce the energy loss arising from the
scattering of the propagating (transmitted) light and to enhance
the optical amplification efficiency.
According to a twelfth aspect of this invention, there is provided
a process for producing a rare earth element-doped optical
waveguide in which a core layer formed of the same material as the
second core waveguide is provided on the low-refractive-index
substrate, and then the first core waveguide is provided on the
thus formed core layer. This process is characterized by the
formation of the core layer formed of the same material as the
second core waveguide, on the substrate, followed by the formation
of the second core waveguide so as to cover the first core
waveguide, whereby the first core waveguide is formed at a central
portion of the core waveguide. Thus, only the central portion of
the core waveguide is doped with the rare earth element. It is
thereby possible to reduce the energy loss arising from the
scattering of the propagating (transmitted) light at the surface of
the core waveguide, and to enhance markedly the amplification gain
and amplification efficiency, as compared to those in the case
where the core waveguide is doped in whole with the rare earth
element.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view showing one embodiment of a rare earth
element-doped optical waveguide according to this invention;
FIGS. 2(a)-(d) shows a set of diagrams representing one embodiment
of the refractive-index distribution in the thickness direction of
the optical waveguide of FIG. 1:
FIGS. 3, 5, 6 and 8 illustrate other embodiments of the rare earth
element-doped optical waveguide of this invention;
FIGS. 4(a)-(d) shows a set of schedule drawings illustrating one
embodiment of the process according to this invention;
FIGS. 7(a)-(d) and 9(a)-(h), 10(a)-(j) and 11(a)-(h) are flow
sheets illustrating another embodiment of the process according to
this invention;
FIGS. 12(a)-(h) shows a set of schedule drawings illustrating an
examplary process according to the prior art;
FIG. 13(a) is an SEM photograph of a rare earth element-doped core
waveguide formed by a process according to the prior art; and
FIG. 13(b) is an SEM photograph of a core waveguide not doped with
a rare earth element.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Some preferred embodiments of this invention will now be explained
referring to the accompanying drawings.
Referring first to FIG. 1, there is shown a rare earth
element-doped waveguide which comprises a substrate 1 (for
instance, a substrate of a semiconductor such as Si and GaAs; a
substrate of a glass such as SiO.sub.2 and SiO.sub.2 doped with a
refractive index controlling dopant; a substrate of a ferroelectric
material such as LiNbO.sub.3 and LiTaO.sub.3 ; or a substrate of a
magnetic material such as YIG), a cladding 2 of a lower refractive
index, n.sub.c, formed on the substrate, and a core waveguide 3 of
a higher refractive index, n.sub.w (n.sub.w >n.sub.c), buried in
the cladding 2. The cladding 2 is formed by using SiO.sub.2 or
using SiO.sub.2 containing at least one dopant, such as B, F, P,
Ge, Ti, Al, Ta, Zn, K, Na, La and Ba. The core waveguide 3 is also
formed by using a material similar to the material for the cladding
2. In a central region .DELTA.T of the core waveguide 3 with
respect to the direction of thickness T, a rare earth element-doped
layer 4 is provided. The dopant for the rare earth element-doped
layer 4 is a dopant containing at least one element selected from
Er, Nd, Yb, Sm, Ce, Ho, Tm and the like. In the case of a
single-mode optical waveguide, the refractive-index difference
between the core waveguide 3 and the cladding 2 is selected in the
range of 0.2 to 0.7%. The thickness T of the core waveguide 3 is
selected in the range of from several micrometers to ten and a few
micrometers, and the width W of the core waveguide 3 is selected in
the range of from several micrometers to ten and a few micrometers.
For stronger confinement of light in the width direction of the
core waveguide 3 and for efficient and concentrated absorption of
excitation light into the region of the rare earth element-doped
layer 4, the core waveguide 3 is so designed that W>T. For
instance, where the optical waveguide is to be used as a
single-mode optical waveguide at wavelength around 1.55 .mu.m, the
core waveguide 3 is so designed that T=7 .mu.m, W=11 .mu.m, the
refractive-index difference between the core waveguide 3 and the
cladding 2 is 0.25%, and .DELTA.T=1 .mu.m to 4 .mu.m. When the
region where the power distribution of excitation light reaches its
maximum in the core waveguide 3, namely, a central portion with
respect to the layer thickness direction of the core waveguide 3 is
thus doped with a rare earth element, it is possible to obtain a
high excitation efficiency through doping with a small amount of
the rare earth element. That is to say, the arrangement in this
embodiment permits the doping with a reduced amount of a rare earth
element, as compared with the doping amount in the conventional
doping where the core waveguide 3 is doped in whole with the rare
earth element, and the arrangement enables a higher excitation
efficiency. Consequently, it is possible to realize a high-gain
optical amplification concentration extinction.
FIGS. 2(a) to (d) each show a refractive-index distribution in the
thickness direction of the rare earth element-doped optical
waveguide shown in FIG. 1. FIG. 2(a) shows the case where the
refractive-index distribution in the thickness direction of the
core waveguide 3 is flat, that is, where the refractive index of
the rare earth element-doped layer 4 is equal to the refractive
index in the rare earth element-undoped region of the core
waveguide 3. FIGS. 2(b) to 2(d) show the cases where the refractive
index of the rare earth element-doped layer 4 is slightly higher
than the refractive index in the rare earth element-undoped region
of the core waveguide 3. More specifically, FIG. 2(b) shows the
case where the refractive-index distribution in the rare earth
element-doped layer 4 is flat, FIG. 2(c) shows the case where the
refractive index increases stepwise as a central portion is
approached, and FIG. 2(d) shows the case where the refractive index
increases continuously in a curved line form as the central portion
is approached. With the refractive index of the rare earth
element-doped layer 4 set greater than the refractive index in the
surrounding core waveguide 3, as shown in FIGS. 2(b) to 2(d), the
confinement of excitation light in the rare earth element-doped
layer 4 is strengthened, and a higher excitation efficiency can be
expected.
In FIG. 3, the cladding 5 is provided in a projected shape so that
the cladding 5 is thinner on the sides of side faces of the core
waveguide 3 than on the other sides, in order to obviate
application of a surplus stress (stress arising from differences in
coefficient of thermal expansion) on the interior of the core
waveguide 3.
The structure of the rare earth element-doped optical waveguide is
not limited to the above embodiment. For instance, a so-called
coupled waveguide structure may be used in which a plurality of
core waveguides 3 is provided in the cladding. Other than the
rectilinear waveguide, there may be used a curved waveguide, a
waveguide having a 90.degree. bent portion and the like.
Furthermore, the waveguide may be combined with other optical
devices (for example, interference filter, lens, prism,
semiconductor laser, detection element). Moreover, the rare earth
element-doped optical waveguide as mentioned above may be used to
construct an optical circuit such as an optical directional
coupler, a Y-junction waveguide, a ring resonator (cavity), an
optical wave demultiplexer, an optical star coupler, an optical
switch, an optical modulation circuit, etc.
FIGS. 4(a) to 4(d) show a set of schedule drawings illustrating one
embodiment of a process for producing a rare earth element-doped
optical waveguide according to this invention.
First, as shown in FIG. 4(a), a first clad layer 6 is provided on a
substrate 1. The first clad layer 6 may be provided by any one of
the CVD method, the flame deposition method, the electron beam
source evaporation method, the sputtering method and the like.
Next, as shown in FIG. 4(b), a first core glass layer 7 is provided
on the first clad layer 6 in a thickness of (T/2-.DELTA.T/2). The
first core glass layer 7 also may be provided by any one of the
above-mentioned methods. Thereafter, a rare earth element-doped
layer 4 is formed on the first core glass layer 7 and, further, a
second core glass layer 8 of substantially the same material as the
first core lass layer 7 is provided on the rare earth element-doped
layer 4. In this process, the first core glass layer 7, the rare
earth element-doped layer 4, and the second core glass layer 8 may
be provided by a continuous process or by a discontinuous process.
Then, as shown in FIG. 4(c), photolithography and dry etching
process are carried out to shape the first and second core glass
layers 7, 8 and the rare earth element-doped layer 4 into a
rectangular form, thereby providing on the first clad layer 6 a
core waveguide 3 having a rectangular cross-sectional shape with
the width W greater than the thickness T. Finally, the core
waveguide 3 is covered by a second clad layer 9 having
substantially the same refractive index as that of the first clad
layer 6.
According to this process, the rare earth element-doped optical
waveguide is provided on the substrate 1 by the planar technique
and, accordingly, a part (rare earth element-doped layer 4) of the
core waveguide 3 constituting the light-transmitting portion of the
waveguide is capable of being doped with the rare earth element
concentratedly and uniformly with respect to the width direction.
By this process, further, it is possible to form the rare earth
element-doped optical waveguide together with other optical devices
in a collective manner on the substrate, and to produce a
multifunctional system with high quality.
Referring to FIG. 5, a plurality of rare earth element-doped layers
are provided in a core waveguide 3. Viewed in the direction of
thickness T of the core waveguide 3, a central region of thickness
.DELTA.T.sub.0, a lower region of thickness .DELTA.T.sub.1, and an
upper region of thickness .DELTA.T.sub.1 are doped with the rare
earth element, to form three rare earth element-doped layers 4
which are spaced apart from each other along the layer thickness
direction of the core waveguide 3. As the rare earth element, an
agent containing at least one of Er, Nd, Yb, Sm, Ce, Ho, Tm and the
like is used. In the case of a single-mode optical waveguide, the
refractive-index difference between the core waveguide 3 and the
cladding 2 is selected in the range of from 0.2 to 0.8%, whereas
the thickness T of the core waveguide 3 is selected in the range of
from several micrometers to ten and a few micrometers, and the
width W of the core waveguide 3 is also selected in the range of
from several micrometers to ten and a few micrometers. For stronger
confinement of light in the width direction of the core waveguide 3
and for efficient and concentrated absorption of excitation light
into the rare earth element-doped regions 10, 11 and 12 of the core
waveguide 3, the core waveguide 3 is so designed that W>T. For
instance, where the optical waveguide is to be used as a
single-mode optical waveguide at wavelengths around 1.5 .mu.m, the
core waveguide 3 is so designed that T=8 .mu.m, W=12 .mu.m, the
refractive-index difference between the core waveguide 3 and the
cladding 2 is 0.259%, .DELTA.T.sub.0 =2 .mu.m, .DELTA.T.sub.1 =1
.mu.m, and the thickness of the region not doped with the rare
earth element (Er) is 1 .mu.m. In this embodiment, an SiO.sub.2
-GeO.sub.2 -B.sub.2 O.sub.3 glass was used as the material for the
core waveguide 3, whereas an SiO.sub.2 -P.sub.2 O.sub.5 -B.sub.2
O.sub.3 glass was used as the material for the cladding 2, and an
SiO.sub.2 glass was used for the substrate 1. When the core
waveguide 3 is doped with the rare earth element in a multilayer
pattern according to the power distribution of excitation light in
the core waveguide 3, it is possible to reduce the doping amount of
the rare earth element, as compared with the amount in the case
where the core waveguide 3 is doped in whole and uniformly with the
rare earth element, and also to enhance the excitation efficiency
of excitation light. That is to say, it is possible to realize a
high-gain optical amplification waveguide free of concentration
extinction.
FIG. 6 shows an embodiment in which a further larger number of rare
earth element-doped layers are provided. In this embodiment, five
layers doped with a rare earth element are provided, spaced apart
from each other along the direction of thickness T of the core
waveguide 3. For T=8 .mu.m, the thickness of the rare earth
element-doped layers 4a to 4e are set to be 1.5 .mu.m, 1 .mu.m, 1
.mu.m, 0.5 .mu.m, and 0.5 .mu.m, respectively. On the other hand,
the thickness of the core layers 3a to 3f not doped with a rare
earth element are set to be 0.75 .mu.m, 0.75 .mu.m, 0.5 .mu.m, 0.5
.mu.m, 0.5 .mu.m, and 0.5 .mu.m, respectively. That is, the
thickness of the rare earth element-doped region is preferably not
more than 30% based on the thickness of the core. Thus, the rare
earth element-doped layers can be provided in from 2 to about ten
and a few layers.
FIG. 7 shows a process for producing the rare earth element-doped
layer optical waveguide of FIG. 5.
First, as shown in FIG. 7(a), a first clad layer 13 is provided on
a substrate 1. The clad layer 13 may be provided by any one of the
CVD method, the electron beam source evaporation method, the
sputtering method, the flame deposition method and the like. Next,
as shown in FIG. 7(b), core layers 10 not doped with a rare earth
element and core layers 11 doped with a rare earth element are
alternately laminated. The core layers 11 may be provided by any
one of the CVD method, the electron beam source evaporation method,
the sputtering method and the like. Subsequently, as shown in FIG.
7(c), photolithography and dry etching process are carried out to
fabricate a multi-layer form core waveguide 3 on the first clad
layer 13. In this case, the rare earth element-doped layers are
capable of being etched easily because they are thin layers. An
etching gas to be used here may be, for instance, CHF.sub.3 or a
mixture of CHF.sub.3 with CHCl.sub.3. Finally, as shown in FIG.
7(d), the core waveguide is covered by a second clad layer 15
having substantially the same refractive index as that of the first
clad layer 13, whereby a buried-type optical waveguide is
formed.
Since the rare earth element-doped layers are provided in a
plurality of layers (two or more layers) in the core waveguide, the
optical waveguide is capable of being used to construct an optical
circuit, such as an optical directional coupling circuit, an
optical wave demultiplexer, an ring resonator (cavity) circuit, an
optical filter circuit and an optical switching circuit, which is
less dependent on polarization.
FIG. 8 shows an optical waveguide in which a core waveguide 3
comprising a first core waveguide 15 substantially rectangular in
cross section and redoped uniformly with a rare earth element and a
second core waveguide 16 so formed as to cover the first core
waveguide 15 and not doped with a rare earth element is provided in
a cladding 2. The optical waveguide is produced by a process
illustrated in FIG. 9.
FIG. 9(a) shows a member obtained by forming a rare earth
element-doped layer on a low-refractive-index substrate 1, followed
by dry etching or the like to form a first core waveguide 15
substantially rectangular in cross section. As the substrate 1
here, a silica glass was used. When a multi-component glass,
sapphire, Si or the like as the material for the substrate, it is
necessary to provide a buffer layer of SiO.sub.2 or of SiO.sub.2
containing at least one refractive index controlling dopant, such
as B, P, Ti, Ge, Ta, Al, F, etc., according to the refractive index
of the core. The first core waveguide 15 contains at least one
refractive index controlling dopant, such as B, P, Ti, Ge, Ta, Al,
F, etc., and also contains at least one of Yb, Er and Nd as a rare
earth element contributing to optical amplification.
If the concentration of Er or Nd reaches or exceeds several
hundreds of ppm at this stage, side faces of the first core
waveguide 15 would show severe roughening upon etching [FIG.
13(a)]. To avoid such a problem, the width W.sub.1 and the
thickness T.sub.1 of the first core waveguide 15 are preliminarily
set slightly smaller than the final core waveguide dimensions, and
is buried completely in a core layer 17 not doped with a rare earth
element and having a refractive index equal to or slightly lower
than the refractive index of the first core waveguide 15, as shown
in FIG. 9(b). Next, as shown in FIG. 9(c), a metal film 18 about 1
.mu.m thick is provided, for the subsequent etching of the core
layer 17. Then, as shown in FIG. 9(d), a photoresist 19 is provided
on the metal film 8 by photolithography. The pattern width W.sub.2
of the photoresist 19 is set slightly (about 2 .mu.m) greater than
the width W.sub.1 of the first core waveguide 15, to enable
sufficient covering of the side faces of the first core waveguide
15. Subsequently, dry etching of the metal film 18, dry etching of
the covering core layer 17, and removal of the photoresist 19 and
the metal film 18 are carried out sequentially, to form on the
low-refractive index substrate 1 a core waveguide 3 which comprises
the first core waveguide 15 and the second core waveguide 16 not
doped with a rare earth element and so formed as to cover the core
waveguide 15. Finally, a clad layer 2 is formed so as to cover the
entire surface of the core waveguide 3.
According to this process, the first core waveguide 15 doped with
the rare earth element is provided and then the second core
waveguide 16 not doped with a rare earth element is provided so as
to cover the first core waveguide 15, and, therefore, it is
possible to provide the core waveguide 3 with a smooth surface.
Consequently, it is possible to prevent the scattering of
propagating (transmitted) light at the surface of the core
waveguide 3. In addition, since the refractive index of the first
core waveguide 15 is equal to or higher than the refractive index
of the second core waveguide 16, light is capable of being
concentrated effectively on the interior of the first core
waveguide 15, and the transmission efficiency of the core waveguide
3 can be enhanced.
FIG. 10 shows another embodiment of the process for producing a
rare earth element-doped optical waveguide. In this embodiment, a
first core waveguide 20 having a refractive index approximate to
the refractive index of a core waveguide 3 and doped with a rare
earth element is provided at a central portion of the core
waveguide 3, in order to contrive enhancement of the performance of
the optical waveguide in use as an optical amplification
waveguide.
First, as shown in FIG. 10(a), a core layer 21 not containing a
rare earth element and a rare earth element-doped core layer 22
having a refractive index approximate to the refractive index of
the core layer 21 are provided in succession on a low-refractive
index substrate 1. Next, by photolithography and dry etching as
shown in FIGS. 10(b) to 10(f), the first core waveguide 20 is
provided. Here, such a patterning is conducted that the width
W.sub.1 and thickness T.sub.1 of the first core waveguide 20
satisfy the relations W.sub.1 =W.sub.2 /3 and T.sub.1 =T.sub.2 /3,
where W.sub.2 and T.sub.2 are the final width and thickness of the
core waveguide. Subsequently, as shown in FIG. 10(g), the first
core waveguide 20 is buried in a core layer 31 having a refractive
inded equal to that of the core layer 21. The resulting body is
again subjected to photolithography and dry etching, as shown in
FIGS. 10(h) and 10(i), to form the core waveguide 3 having the
width W.sub.2 and the thickness T.sub.2. Finally, a clad layer 2 is
provided, as shown in FIG. 10(j).
A process shown in FIG. 11 is forming a low-loss rare earth
element-doped waveguide and a rare earth element-undoped waveguide,
in a collective manner on the same substrate. The steps shown in
FIGS. 11(a) to 11(c) are the same as those shown in FIGS. 9(a) to
9(c). In this process, however, in the next step of FIG. 11(d), a
photoresist 19 is patterned also at a portion other than the
portion directly above the first core waveguide 15, whereby the
rare earth element-undoped core waveguide 24 can also be formed
together. The steps shown in FIGS. 11(e) to 11(h) are the same as
those shown in FIGS. 9(e) to 9(h). According to this process, for
instance, an Er-doped optical waveguide to be used for a signal
optical amplifier at a wavelength of 1.53 .mu.m and an optical
waveguide for excitation light at a wavelength of 1.46 to 1.48
.mu.m are capable of being formed on the same substrate in one
body.
* * * * *